This disclosure relates generally to electrocardiogram (ECG) monitoring. More particularly, this disclosure relates to implementation of the front end of an ECG monitor.
As is known, the cardiac cycle can be measured non-invasively by attaching small electrodes on the skin of the patient. The voltage differences caused by the heart between the electrodes are measured and recorded to obtain the electrocardiogram (ECG) of the patient. ECG electrodes may also be used to measure a respiratory signal from the patient. For this, a high frequency excitation signal well above the ECG frequencies is normally supplied to the patient. Thus, although filters are used to reduce environmental interferences, certain frequencies higher than the actual ECG frequencies have to be passed to the monitor. In order to be able to record the weak voltages in all circumstances for all patients, the measurement is faced with many challenges. To meet the challenges, the complexity of the front end inevitably increases, if too many compromises are not to be made.
In practice, the ECG measurement electronics needs to be protected from the high voltage pulse of a defibrillator, as the defibrillator pulse may cause permanent damage to the ECG measurement apparatus. Furthermore, the protection is to be implemented so that the energy of the defibrillator pulse is not shunted by the measurement apparatus, which would lead to ineffective defibrillation. The ECG measurement apparatus, i.e. the ECG monitor, should also be capable of detecting pacemaker spikes from the measured waveform signals and filter out the noise and interference caused by other equipment used to treat the patient, such as an electrosurgery unit. An example of the contradictory requirements is that the electrosurgery unit requires the use of a filtering capacitor to filter out noise, while the presence of such a filter is not desirable in view of detection of the fast pacemaker spikes or other high frequency signals, such as the excitation signal. Unless filtered away, the high frequency noise generated by the electrosurgery unit is downconverted in a clamp circuit required to shunt defibrillator currents. The clamp circuit is normally needed, although it produces additional (low frequency) noise in presence of unfiltered high frequency noise from e.g. an electrosurgery unit.
The contradictory requirements have led to an implementation in which the protection resistors form parallel and dedicated input branches for the actual ECG signal on one hand and for the pacemaker spikes and respiratory signals on the other hand. In each input branch the protection resistor drops the defibrillator output voltage, which is of the order of 5 kV, to a level of about 5 volts or below. Considering that a well-designed ECG front end includes 2 or 3 protection resistors for each ECG electrode, the total number of protection resistors becomes rather high. This translates to high space requirement, cumbersome practical circuitry, and high cost. Furthermore, increased number of protection resistors results in increased number of parallel input branches, which tends to increase the input impedance imbalance of the front end circuitry. This, in turn, detracts from the ability of the circuitry to reject common mode noise.